Great care must be taken when using herbicides in young papaya plantings. The papaya plant does not produce woody bark as is found in most trees. The papaya trunk is able to absorb herbicide and this can lead to damage to the trunk. While the trunk is still green or has only started to become brown the plant is at its most susceptible stage.
During some conditions, particularly warm wet weather, damage to the trunk can occur which appears somewhat similar to that caused by the fungus, Phytophthora sp.. Leaves wilt on affected plants and whitish exudates may be present on the trunk. The only way to confirm the cause of the damage is to put samples through a plant pathology laboratory where tests can confirm the presence/absence of the fungus.
The use of the herbicides glufosinate ammonium and paraquat can lead to basal stem rots, which may partially or fully girdle the trunk of the plant. The damage appears initially as corky darker coloured tissue with vertical fissures, but can develop into soft spongy moist lesions, sometimes with white sap exudates present.
These trunk lesions result in restricted expansion of the trunk at the site of the damage, which can lead to collapse of the trunk at this site or snap off in heavy rain or windy conditions.
Photo 1. Herbicide damage (glufosinate ammonium) to young papaya trunks showing stem constriction and collapse just above the soil surface.
Photo 2. Close up of a herbicide damaged trunk showing white sap (=latex) exudates, which may be confused with the white spore masses, produced by Phytophthora fungus.
Photo 3. Collapsed trunk due to Phytophthora infection that appears somewhat similar to herbicide damage.
Damage may be reduced or avoided by using shielded spraying equipment so that herbicide contact with papaya trunks is minimised. The use of mulches or films reduces the need to treat weeds. Pulling weeds away from the papaya trunks prior to spraying and careful application of sprays will also reduce the risk of damaging the trunks. Herbicides should always be applied at label rates and concentrations. There appears to be some variation between papaya varieties in their susceptibility to herbicide damage.
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I would like to thank plant pathologists, Mr. Lynton Vawdrey and Ms. Kathy Grice, DAFF, Qld for assisting with information and diagnostics of samples. Ms. Delina Connell (Scientific Advisory Services) assisted with production and upload of the article to web. I also wish to thank the growers who allowed me access to their properties to inspect and photograph their plants.
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